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Transitional Tholeiitic Basalts in the Tertiary Bana Volcano–Plutonic Complex, Cameroon Line

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Transitional Tholeiitic Basalts in the Tertiary Bana Volcano–Plutonic Complex, Cameroon Line Journal of African Earth Sciences 45 (2006) 318–332 www.elsevier.com/locate/jafrearsci Transitional tholeiitic basalts in the Tertiary Bana volcano–plutonic complex, Cameroon Line Gilbert Kuepouo a,b,*, Jean Pierre Tchouankoue b, Takashi Nagao c, Hiroaki Sato a a Graduate School of Science and Technology, Department of Earth and Planetary Sciences, Faculty of Science, Kobe University, Nada, Kobe 657-8501, Japan b Department of Earth Sciences, Internal Geodynamics Laboratory, University of Yaounde-I, PO Box 812, Yaounde, Cameroon c Center for Instrumental Analysis, Yamaguchi University, 1677-1 Yoshida, Yamaguchi 753-8512, Japan Received 10 May 2005; received in revised form 7 March 2006; accepted 9 March 2006 Available online 18 May 2006 Abstract The Bana transitional tholeiitic basalts occurring in a Tertiary volcano–plutonic complex of the Cameroon Line, Central Africa are plagioclase-bearing and olivine-free. K/Ar dating on separated plagioclases of the transitional tholeiitic basalts yields an Oligocene age of 30.1 ± 1.2 Ma. Their clinopyroxene compositions are marked by iron enrichment and calcium depletion in the Wo–En–Fs system. The whole-rock major element compositions are characterized by Mg# 36–48, normative quartz and hypersthene. The youngest alkali bas- alts from the same igneous complex have higher Mg# 56–66. These two groups of basalt have trace element characteristics of within- plate basalt with Zr/Nb ratios of 3.7–4.5 and 7.5–9.2 respectively, and different LILE/HFSE and LREE/HREE ratios. The overall trace element characteristics suggest that the transitional tholeiitic basalts of the Bana complex were derived by high degrees of partial melting in the upper mantle at shallow depths whereas younger alkali basalts in the complex were probably produced by a small degree of melting of the same source at slightly greater depths. The transitional tholeiitic character of these basalts suggests a significant lithospheric exten- sion and mantle upwelling below the Cameroon Line in the Oligocene. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Transitional tholeiitic basalts; WPB; Tertiary Bana volcano–plutonic complex; Cameroon Line 1. Introduction uted to the understanding of geochemical and isotopic fea- tures of alkali basalts (Fitton and Dunlop, 1985; Fitton, Oceanic and continental lavas constitute the extrusive 1987; Halliday et al., 1988, 1990; Lee et al., 1994; Marzoli section of the Cameroon Line (Fig. 1), a line of Eocene– et al., 2000; Rankenburg et al., 2005). These studies high- Oligocene anorogenic volcano–plutonic complexes, and lighted the chemical and isotopic similarities between bas- Oligocene to Present volcanic centers. alts from oceanic and continental sectors, their alkaline The Cameroon Line comprises about 60 anorogenic vol- nature and the dominance of the HIMU mantle source cano–plutonic complexes, and a large number of polyge- during their genesis (Halliday et al., 1990). netic and monogenetic volcanoes extending from Pagalu De´ruelle et al. (1991) showed that extensive petrological Island in the Atlantic Ocean (SW) to lake Chad (NE) on studies of the Cameroon Line volcanic and plutonic rocks the Africa continent. A number of studies of continental do not allow the conclusion that magmatism develops a and oceanic basalts of the Cameroon Line have contrib- transitional trend in that region as is the case in the East- African Rift. They discarded a transitional character of some basalts from Mt Oku, Manengouba and Principe * Corresponding author. Address: Department of Earth Sciences, Internal Geodynamics Laboratory, University of Yaounde-I, PO Box island (Fitton, 1987), and gabbros from the Mboutou 812, Yaounde, Cameroon. Tel.: +237 720 22 71. igneous complex (Parsons et al., 1986) on the basis of their E-mail address: [email protected] (G. Kuepouo). high niobium contents compared with typical transitional 1464-343X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2006.03.005 G. Kuepouo et al. / Journal of African Earth Sciences 45 (2006) 318–332 319 Fig. 1. Geologic map of the Bana plutono–volcanic complex (Kuepouo et al., 2004). Inset shows the position of the Bana volcano–plutonic complex in the Cameroon line. basalts of the East-African Rift (Kampunzu and Mohr, basalts, and typified by their mildly hypersthene-normative 1991) and OIB. Further record of transitional basalts to mildly nepheline-normative character. within the Cameroon Line by some workers (Kampunzu The objectives of this paper are: (1) to prove the occur- and Lubala, 1991; Moundi et al., 1996; Moundi, 2004; rence of transitional tholeiitic basalts discovered within the Fosso et al., 2005) is hotly debated. Cameroon Line; (2) to discuss their clinopyroxene and For clarification, transitional basalts are basalts having whole-rock chemistries in comparison with those of typical compositions intermediate between tholeiitic and alkaline alkali basalts of the Cameroon Line occurring in the same 320 G. Kuepouo et al. / Journal of African Earth Sciences 45 (2006) 318–332 complex; and (3) to define and understand the genetic rela- Previous petrological studies of basalts along the Cam- tionship between these two groups of basalts in this setting. eroon Line showed that they were derived from a depleted asthenospheric source beneath the subcontinental litho- 2. Geological background sphere (Halliday et al., 1988; Sato et al., 1990), or from amphibole-bearing lithospheric mantle for the continental The Tertiary Bana volcano–plutonic complex (5°80S, basalts (Marzoli et al., 2000). Geophysical studies revealed 5°110N; area ca. 55 km2) is located southeast of the West that the Cameroon Line is underlain by a thin crust of Cameroon Highlands in the central part of the Cameroon ca. 30–34 km thickness (Fairhead and Okereke, 1987; Line (Fig. 1). A previous petrological study on the Bana Plomerova et al., 1993; Poudjom Djomani et al., 1995). complex (Nana, 1988) distinguished: (1) the plutonic Previous hypotheses on the origin of the Cameroon Line unit including a biotite ± amphibole granite, an arfvedso- are documented elsewhere (Moreau et al., 1987; De´ruelle nite ± aegirine granite, and a small lens of leucogabbro; et al., 1991; Burke, 2001). Recent studies by Poudjom (2) the volcanic unit made up of small lava flows, basaltic Djomani et al. (1997) suggest that the Cameroon Line fol- lapilli tuffs and rhyolitic cinder tuffs forming a ring around lows a major structural zone in the lithosphere. and on top of the arfvedsonite ± aegirine granite at the North. 3. Petrography Attempts to date the Bana complex by the K/Ar method resulted in values that ranged from 38 ± 1 and 42 ± 8 Ma 3.1. Plagioclase-basalts on benmoreite lavas (Cantagrel et al., 1978; Nana, 1988) and 30 Ma (no analytical precision reported) on ‘‘evolved The plagioclase-basalts from Bana are subaphyric to lava’’ (Lasserre, 1978). The arfvedsonite ± aegirine granite porphyritic with a primary mineral assemblage dominated yielded an Rb/Sr age of 51 ± 1 Ma and an initial 87Sr/86Sr by plagioclase, Fe–Ti oxides and clinopyroxene. In plagio- ratio of 0.7035 ± 0.0001 (Caen-Vachette et al., 1991). clase-phyric basalts, plagioclase phenocrysts >3 mm across A detailed geologic map (Fig. 1) of the Bana volcano– represent ca. 30–50 vol.%. Large plagioclase phenocrysts plutonic complex is available from Kuepouo (2004). The range from 5 to 15 mm in length. Individual phenocrysts Bana complex forms a prominent mountainous scarp cul- are euhedral to subhedral, although strongly corroded minating at 2097 m above sea level, rising 600 m above crystals were observed. Glomeroporphyritic associations the general level of the countryside to the south and show cruciform intergrowth of plagioclase. The intersertal 350 m to the northwest. This complex is bounded to the to intergranular groundmass is dominated by plagioclase south by an elevated (up to 1500 m) crystalline basement microlites. Phenocryst and microlite compositions vary encompassing Neoproterozoic granite and gneisses, cross from bytownite to andesine. Interstitial minerals are cut by mafic and felsic dykes. Aerial photographs and field Fe–Ti oxides, apatite needles and occasionally albite and observations reveal a sharp discordant contact between the accessory titanite. Plagioclase phenocrysts are partly saus- complex and the basement to the south. suritized in altered samples and the groundmass partly The complex includes two crescent-like plutonic units replaced by micas, epidote and carbonate. Chlorite replaces forming the ‘‘Southern Intrusions’’ and the ‘‘Northern glass. Intrusions’’ (Fig. 1). From west to east, the Southern Intru- Clinopyroxene constitutes 7.6 vol.% mostly as microlites sions consist of biotite-amphibole granite and biotite (<0.3 mm) and occasional microphenocrysts (up to 1 mm miarolitic granite, whereas the Northern Intrusions are across). Microphenocrysts form isolated grains in the made of arfvedsonite-eckermannite ± aegirine granite and groundmass or inclusions in plagioclase phenocrysts, and syenodiorite and quartz-syenodiorite. Syenodiorites of the are occasionally chloritized. In the groundmass of porphy- outer margin include several alkali feldspar and quartz ritic basalts, small grains of clinopyroxene fill the intersti- crystal basement xenoliths. tial space between plagioclase laths. The volcanic rocks include lava flows and pyroclastic Fe–Ti oxide microphenocrysts (titanomagnetite and rocks exposed in the center and north of the complex. ilmenite) represent
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